Neurosurgical interventions are emerging as treatments for a variety of intractable neurological conditions, including among them, movement disorders, pain, and epilepsy. Different modalities of treatment that target discrete anatomical sites are in current use or in development, including radiofrequency lesioning, chronic electrical stimulation, tissue implantation, and microdialysis. The strategy of these interventions is to diminish or enhance the activity of these sites so as to produce a therapeutic effect. In all cases, accurate targeting is essential to obtain an optimal treatment with minimal risk to the patient.
Targeting is often effected through devices that establish a neural interface. Such devices are important for clinical and scientific purposes. A ‘neural interface’ refers to the interface between a device and a targeted region of the nervous system for the purposes of recording neural signals, stimulating neurons, and delivering fluidic agents, or combinations of these purposes. A ‘neural interface region’ refers to the volume of the nervous system that is recorded from, stimulated, or affected by delivery of the fluidic agent. In general, neural interface regions could extend from as small as 1 micron or less from the device surfaces to several centimeters from the surfaces, depending on the application. ‘Tuning’ the neural interface region refers to selectively adjusting the recording, stimulation, or fluid delivery regions to target specific neural structures.
As only one example, a current method for targeting for placement of a deep brain stimulation (“DBS”) electrode for Parkinson's Disease involves the use of neurophysiological (functional) mapping of structure boundaries with reference to high quality CT or MRI images of the brain. At present, mapping involves penetrating the computed target structures with single-channel movable wire microelectrodes to identify the neuronal structure boundaries. Each microelectrode is advanced very slowly, stopping to examine individual cells and to record the firing frequency and pattern. When the microelectrode reaches the target brain structure, a typical change in electrical activity occurs, due to the sustained pattern of discharge of specific neuron types.
In addition to using electrical recording techniques for mapping, macro- or microstimulation (from the same or a second adjacent electrode) can be used to assess the effects of electrical stimulation on units along the trajectory and in the potential target. Specifically, combined microrecording and microstimulation techniques can be used to physiologically determine the location of both target and non-target areas deep within the brain. Due to limitations related to the electrode, which is typically a single-channel device or a number of single-channel devices used together, this mapping procedure is often tedious, time-consuming, and difficult, which combine to limit its utilization and effectiveness.
Conventional single-channel electrodes are typically formed from small diameter metal wires (e.g., tungsten, stainless steel, platinum). These electrodes are most often formed by electrolytically sharpening or mechanically beveling the wire to a fine tip (<1 micrometer) and then insulating it, leaving only the tip exposed. Alternatively, microwires can be formed from pre-insulated fine wires that have been cut to expose the cross-sectional area at the end of the wire. These types of wire devices can be converted into electrode arrays by combining multiple wires into an assembly. However, this bundled wire construction creates limitations to establishing selective neural interfaces because the size, number, and location of electrode sites are intricately related to the device's size, shape, stiffness, and structural complexity. Such devices are also difficult to manufacture to small tolerances on the order of an electrode site feature size, typically in the range of 1-15 microns. The relatively large variability in electrode size and the limited electrode array configurations of these devices preclude the ability to connect groups of electrodes together to selectively tune the neural interface.
As an alternative to bundled wires, multichannel electrode arrays that utilize wafer-level microfabrication methods employed in the semiconductor industry have been under development for nearly three decades (Wise, et al., 2004, Proc. IEEE, 92:72-97) and have been used for neurophysiological research. In general, these techniques are similar to those used to create integrated circuits and utilize similar substrate, conductor, and insulating materials. Fabrication typically starts on a wafer substrate, and the electrode features are added using a number of photolithographically patterned thin-film layers that are defined by etching. These methods are attractive since they result in reproducible, batch-processed devices that have features defined to within less than +/−1 micron. Using these methods, an individual electrode site can be made about as small as the tip of a small wire microelectrode, while the microelectrode shank, the portion that supports the electrode sites and displaces the tissue, can carry multiple recording sites and can cross-sectional area and volume that is comparable to a single wire electrode.
Generally speaking, the fabrication processes for current microfabricated devices impose practical limits on the length of the device to about less than 1 cm. Such a device would not be suitable, for example, for a human deep brain mapping electrode which must penetrate at least 70 mm from the brain surface to target the basal ganglia or thalamus. In addition, these mapping electrodes require extra length of up to 200 mm for mounting in a stereotactic frame and for connection to external instrumentation. Another limitation to microfabricated devices is that the materials typically used as a substrate are often either too brittle (e.g. silicon) or too flexible (e.g. polyimide, parylene) to precisely target particular neural structures.
There are examples of multi-site devices for neural interfacing that do not use wafer level microfabrication techniques but that do employ similar processing steps such as thin-film metal deposition and subsequent laser micromachining to define electrode traces on a central core. While these devices may provide a high density of sites on a substrate similar in size to conventional single-channel wires, and they can be manufactured on substrates that can target human deep brain, there are several desirable characteristics that are absent. First, the devices are not batch fabricated. In other words, each device must be treated individually to form a plurality of electrode interconnects and sites. Additionally, in many cases, each interconnect as well as each site must be individually formed. Second, the array of electrode sites and interconnects are built up from the structural substrate in a manner that generally makes the formation of the electrical features (e.g. sites, traces and connection contacts) closely dependent on the length, shape, and material properties of the substrate. The coupling between formation and placement of the electrode sites and the underlying structural component limits the ability to group small sites to form a macro site to shape the interface range.
There exist examples of devices used for clinical deep brain mapping, which are designed for intraoperative use only. These consist of a single microelectrode site that is appropriate in size for neurophysiological mapping. While these electrodes have enabled improved placement of deep brain stimulation electrodes, they only offer recording capability at the tip, limiting the capability to tune the region with which the device is interfaced.
There are also examples of devices used for clinical DBS, which are designed for long-term implantation and functionality. These devices are comprised of a flexible polymer cylindrical substrate with four metal electrode contacts (sometimes referred to as “macroelectrodes”). These electrode contacts are configured such that each electrode site is placed around the perimeter of a flexible substrate to form a cylindrical shape. The electrode sites are positioned linearly along the axis of the cylindrical substrate. Due to the relatively large size of this device (including its stimulating surfaces), the small number of stimulating sites, and the way that it is constructed, this device is limited in its ability to establish tunable neural interface regions.
The ability to record and/or stimulate, for example, through multiple electrode sites simultaneously has the potential to greatly improve the speed and accuracy of the mapping procedure. While single site electrodes are limited by permitting recording from only a single point in tissue at a time, electrodes with multiple spatially separate recording channels would be capable of recording simultaneously from many points. Recordings may be comprised of spontaneous neuronal activity, movement-related activity, or evoked activity as a result of stimulation from nearby sites. Simultaneously sampled recordings could be exploited to increase the speed and accuracy by which data are acquired. Electrode arrays that are capable of simultaneously sampling from the same neuronal region are also likely to detect regions of statistically independent background noise and/or artifacts. Using advanced signal processing techniques such as independent component analysis, these unwanted signals could be identified and removed, resulting in improvement of the signal-to-noise ratio, and in turn facilitating neuronal spike discrimination. This technique may also reveal signals that were previously hidden within the background noise. Thus, an unmet need remains for a neural interface device that:                Can be configured to create selective and tunable neural interface regions over large spatial distances;        Establishes high-resolution multi-site interfaces targeted at all regions of the nervous system (e.g., centrally or peripherally), including deep brain regions;        Establishes multi-modal (e.g., electrical and chemical) interfaces targeted at all regions of the nervous system (centrally or peripherally), including deep brain regions;        Has a large design space (e.g., site area, site spacing, substrate shape) to provide customizable devices specific to a variety of applications;        Is capable of supporting a high density of electrode sites on a substrate/carrier that is the same size or smaller than conventional single-channel microelectrodes;        Is fabricated from biocompatible materials; and        Is easily manufactured.        